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Quaternary Deposits Near the San Emigdio Mountains, California: Evidence for a Paleolandscape?

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Quaternary Deposits Near the San Emigdio Mountains, California: Evidence for a Paleolandscape? QUATERNARY DEPOSITS NEAR THE SAN EMIGDIO MOUNTAINS, CALIFORNIA: EVIDENCE FOR A PALEOLANDSCAPE? By: Paul G. Lavelle Dr. Antonio F. Garcia Advisor Earth and Soil Sciences Department California Polytechnic State University San Luis Obispo 2006 QUATERNARY DEPOSITS NEAR THE SAN EMIGDIO MOUNTAINS, CALIFORNIA: EVIDENCE FOR A PALEOLANDSCAPE? Paul G. Lavell e November 2006 ABSTRACT Discontinuous low-relief surfaces are scattered throughout relatively high topography within the San Emigdio Mountains, California. These surfaces are considered anomalous, as they are preserved in a dissected, mountainous region that is affected by ongoing orogeny. Previous research has suggested that the low-relief surfaces may represent a once-contiguous alluvial surface. This project utilizes field mapping and scdimentological analysis to determine if the surfaces represent a paleo landscape. What is apparent from field work is the presence of two morphologically distinct lithologie units that most likely represent surficial geologic components of the same relict landscape. Acknowledgments I would like to thank Dr. Tony Garcia for all of his generous help and support throughout my career at Ca l Poly. Dr. Garcia’s teaching efforts have been inspirational. I would also like to thank Trisha, and my parents Robert and Deborah, for always being there for me. Approval Page TITLE: Quaternary deposits near the San Emigdio Mountains, California: evidence for a paleolandscape? AUTHOR: Paul G. Lavelle DATE SUBMITTED: December, 2006 Dr. Antonio F. Garcia Senior Project Advisor Signature Dr. Brent Hallock Department Chair Signature iii TABLE OF CONTENTS ABSTRACT.......................................................................................................... i ACKNOWLEDGMENTS....................................................................................... ii APPROVAL PAGE............................................................................................... iii TABLE OF CONTENTS........................................................................................ iv LIST OF FIGURES............................................................................................... v LIST OF TABLES................................................................................................. .vi INTRODUCTION.................................................................................................. 1 REGIONAL GEOLOGY AND TECTONICS......................................................... 2 MATERIALS AND METHODS............................................................................. 4 RESULTS.............................................................................................................. 6 DISCUSSION........................................................................................................ 9 CONCLUSIONS................................................................................................... I I REFERENCES...................................................................................................... 13 APPENDICES........................................................................................................ 14 A. Figures ................................................................................................. 14 B. Sedimentological descriptions of map units ................................... 17 C. Tables ................................................................................................... 19 D. Surficial geologic map of the Apache Saddle / Bitter Creek environs, San Emigdio Mountains, Kern and Ventura Counties, California ..................................................................... 20 LIST OF FIGURES Site Map................................................................................................................ 14 Low Relief Surface (photograph) ........................... ............................................................ 15 Low Relief Surface (photograph) ................................................. ....................................15 Exposure of map unit Qcf (photograph) .......................................................... ................... 16 Exposure of map unit Qaf (photograph)... .................................................................... 16 v LIST OF TABLES 1. Field criteria for distinguishing facies types on alluvial fans Introduction This study focuses on prominent low-relief surfaces preserved above 900m near the San Emigdio Mountains. California, field observations and data collected will comprise one phase of a larger-scope investigation evaluating the influence of climate, erosion and tectonics in a region affected by Quaternary upli ft. Hypsometric analysis of the study area has shown evidence for a once contiguous low-relief alluvial surface (Dean, 2005). If the alluvial deposits comprising the surface represent parts of a paleolandscape, they may convey information regarding geomorphic processes. Similar plateau-like surfaces have been used in previous studies to deduce climate- erosion-tectonic effects. Researchers tend to diverge on whether climate or tectonics is the major driving force in a given landscape. In an area of constant uplift, Grujie (et al., 2006) revealed that climate-induced decreases in erosion rates caused the upli ft and preservation of a paleolandscape. In a separate study of deep tectonic processes, Rogers (et al., 2002) proposed uplift of a large- scale plateau surface occurred as an upper plate response to the influx of mantle asthenosphere. The objective of this project is to describe and map the geology of prominent low-relief surfaces in the project area. The project area is in the northern San Emigdio Mountains in Southern California (Figure 1 ). The surfaces are geographically between the uplifted San Emigdio Mesa and northern San Emigdio Mountain fronts (Photos; Figures 2-3; p. 15). Mapping boundaries range north to south from California Highway 166, to the Apache Saddle Fire Station (Appendix D). The anomalous topography was studied through field reconnaissance, identification of the surfaces, field-geologic mapping, sedimentologica l description, and stereoscope analysis of air photos. It is proposed that the deposits constitute remnants of a paleolandscape. 1 Regional Geology and Tectonics The most prominent active structural feature within the study area is the right- lateral San Andreas Fault. The San Andreas Fault System is 1300km long, extending from the Mendocino Fracture Zone to the East Pacific Rise as a complex zone of sheared rock varying from 0.5 to 1km in width (Wallace, 1990). Since 29 Ma, the fault has produced a highly complex pattern of rock distribution and fault strands (Wallace, 1990). Within the project area the San Andreas Fault strikes N 60° W in what is known as the Big Bend portion of its trace (Wallace. 1990). The fault has a documented overall slip rate of 20-30mm/year, but the segment within the project area is considered locked at present (Irwin, 1990). Other active structures near the San Emigdio Mountains include (1) the White Wolf Fault, with an undetermined magnitude of left slip and approximately 5km of vertical separation; and (2) the Pleito Fault System, a series of east-west trending, south-dipping thrust faults that are the consequence of north-south shortening in the Transverse Ranges (Keller et al., 2000; after Rodgers and Chinnery, 1973 and Working Group on Cali fornia Earthquake Probabilities, 1995). The San Emigdio Mountains cut across the structural grain of California trending east-west, as part of the southern Coast Ranges (Keller et al., 2000). Since the late Cenozoic, rocks of the San Emigdio Mountains have been uplifted ~7km to their present elevation of ~2130m (Keller et al., 2000). The San Emigdio Mountains form the boundary between the southern San Joaquin Valley and the Transverse Ranges. As shown by Keller (et al., 2000), the uplift rate in the northern front of the San Emigdio Range is approximately 2.7-4.3 m/ky, and the locus of active uplift is shifting progressively basinward (northward) toward the San Joaquin Valley. The uplifted Transverse Ranges block is widening as deformation shifts basinward, causing a series of active and relict mountain fronts to emerge (Keller et al., 2000). Due to relatively shallow (<3 km) thrust faulting associated with the Pleito Fault System, vertical displacement is transferred into marked surface uplift (Keller et al., 2000). The northern San Emigdio Mountain fronts are primarily underlain by Bitterwater Creek Shale, Monterey Shale and Santa Margarita Formation conglomerate (Dean, 2005). The latest Tertiary and Quaternary deposits (the oldest being Pliocene San Joaquin Formation) found at the mountain fronts consist predominantly of sandstone and j A conglomerate beds with clasts derived from older rocks found in the adjacent mountain block (Keller et al., 2000). Formative geology of the San Emigdio Mountains includes igneous and metamorphic rocks that are overlain on the northern flank of the range by thick Cenozoic strata (Tulare Formation) (Keller et al., 2000). 3 Materials and Methods Field reconnaissance of the study area was performed prior to mapping hy traveling along Cerro Noroeste Road, north of Apache Saddle. This was done to assess the regional extent of low-relief surfaces legally accessible on foot. USGS-NAPP infrared aerial photographs (Project Designation 1884; images 51-198. dated 6-11-1989; and Project Designation 1892; images 173-189, dated 6-18-1989)
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